C++: Tidy up 1.24 change notes
C++: Tidy up 1.24 change notes

- Merged the two notes for `cpp/uncontrolled-allocation-size` into one.
- Added note about renaming of a query id.
- Moved the use of IR in queries from the library section to the queries
  section, rephrasing the note in terms of query results/performance
  rather than library implementation.
- Grouped, without text changes, the three notes about the `Allocation`
  library
- Grouped all the notes about standard-library models, abbreviating them
  to eliminate the common text.
- Removed the note about `strlen` (#2647) since that should no longer
  affect the results of queries or IR data flow after we started using
  unsound IR for data flow.

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